Key facts about West Nile disease (CDC 2019)

West Nile virus (WNV) is the leading mosquito-borne disease in the continental United States.  It is most commonly spread to people by the bite of an infected mosquito. Cases of WNV occur during mosquito season, which starts in the summer and continues through fall. There are no vaccines to prevent or medications to treat WNV in people. Fortunately, most people infected with WNV do not feel sick. About 1 in 5 people who are infected develop a fever and other symptoms. About 1 out of 150 infected people develop a serious, sometimes fatal, illness. You can reduce your risk of WNV by using insect repellent and wearing long-sleeved shirts and long pants to prevent mosquito bites.

The history of WNV in the USA

In 1999 a significant outbreak of WNV neuroinvasive disease was identified in the New York city area. A coincident die off of corvid birds served to further heighten concern. Initially, St. Louis encephalitis was suspected, but tests on the birds confirmed West Nile Virus (WNV). Until then, WNV had not been confirmed in the Western hemisphere.

New York public health officials understood the threat posed by WNV. They implemented mosquito eradication programs, fearing that WNV would spread as the birds migrated south. Those fears proved to be well founded.  Despite their efforts, the disease spread south, north and west as well.

By the end of 1999, WNV had been detected in 4 states; by 2000, it had spread to 8 states; and by 2001, to 22 states and Ontario, Canada. In 2002, the greatest number of cases were in Illinois; in 2003, in Colorado; and in 2004, in California, showing the inexorable march of the infection across the United States.

The CDC recommends that West Nile virus (WNV) disease should be considered in any person with a febrile or acute neurologic illness who has had recent exposure to mosquitoes, blood transfusion, or organ transplantation, especially during the summer months in areas where virus activity has been reported (CDC).

Instant feedback:
WNV has spread quickly since 1999, infecting thousands of people in North America.


CDC. West Nile Virus. Home. For Health Care Providers. Clinical Evaluation & Disease. Diagnosis & Reporting.
Retreived 11/9/17.

CDC. West Nile virus. Statistics and Maps. Retrieved 6/9/2019

Kent R, Juliusson L, Weissmann M, Evans S, Komar N. (2009) Seasonal blood-feeding behavior of Culex tarsalis (Diptera: Culicidae) in Weld County, Colorado, 2007. J Med Entomol. 46(2):380-90.